Numerous social and political theorists have referred to social groups or societies as “unities.” What makes a unity of a social group? I address this question with special reference to the theory of social groups proposed in my books On Social Facts and Living Together: Rationality, Sociality and Obligation. I argue that social groups of a central kind require an underlying “joint commitment.” I explain what I mean by a “joint commitment” with care. If joint commitments in my sense underlie them, what kind of unity does this give social groups? In what sense or senses is it objective?